GB2043665A - Polyurethanes having adhesive properties - Google Patents

Abstract

A process for preparing a polyurethane, which comprises reacting together, in the absence of a solvent, a diisocyanate; a first diol, having an average molecular weight of between 500 and 5000; at least three diols having a molecular weight below 500, one of which has melting point- lowering characteristics and another of which is unbranched and free of ether groups; and 0 to 5 mole %, based on the equivalent total weight of the low molecular weight diols, of a monofunctional compound; in which the equivalent ratio of the melting point-lowering diol to the unbranched diol is from 20:80 to 75:25. The products are suitable for use in fusible interlinings.

Description

SPECIFICATION Polyurethanes having adhesive properties Fusible interlinings are known. Suitable thermoplastic materials may be applied as fusible adhesives to flat textile structures, for example by applying thickened pastes or solutions with a doctor blade or by sprinkling on powders. It is also known to spray fusible adhesives on in the form of dispersions or solutions or to apply the adhesive material in the form of non-woven filamentary fabrics. On subsequent hot ironing, fabrics sandwiching the adhesive are bonded. The resultant bonds can be resistant to dry-cleaning and washing and need not affect the soft textile handle.

Suitable thermoplastic materials include polyethylenes, polyvinyl chlorides and polyamides. Highpressure polyethylenes have low resistance to dry-cleaning since they swell or dissolve in solvents such as perchloroethylene. Low-pressure polyethylenes are resistant to cleaning but require stringent fixing conditions which preclude their application in many cases, particularly on sensitive fabrics.

Polyvinyl chloride is not wholly suitable as a fusible adhesive since, for this purpose, large amounts of plasticiser must be added, and this latter constituent may migrate or creep, causing a reduction in the separation resistance of a material bonded with the adhesive and a more brittle product. Further, polyvinyl chloride plastisols require fixing conditions which are no longer acceptable.

While polyamides have found popularity as adhesives, they are only suitable in some fields because of their hardness.

Polyurethanes are known as fusible adhesives but generally have considerable disadvantages, including undesirably high melting/softening points, inadequate adhesive strength, and undesirable high hardness.

Known polyurethanes have no great advantages compared with other known fusible adhesives.

Recently, increasingly thin and sensitive fabrics have come onto the market and which have proved difficult to fix with any conventional fusible adhesives. For example, raincoat fabrics provided with a polyurethane coating over the whole of one face have proved particularly difficult to handle. Fabrics finished in this way give a bond which is of minimal or insufficient strength when polyethylenes, polyamides or polyvinyl chloride/polyvinyl acetate copolymers are used as fusible adhesives.

It is known to prepare polyurethanes by converting isocyanate group-containing prepolymers to products of high molecular weight and a correspondingly high melting point using chain lengtheners. Polyurethanes having low melting points are also known but generally have low strength. It would be desirable to provide a strong low melting point polyurethane suitable for use as a fusible adhesive in interlinings.

According to the present invention, a process for preparing a polyurethane comprises reacting together, in the absence of solvent, polyurethane-forming components having a characteristic number of from 96 to 100 and comprising a diisocyanate; a first diol having an average molecular weight of between 500 and 5000; at least three diols having a molecular weight below 500, one of which has melting point-lowering (MPL) characteristics and another of which is unbranched and free of ether groups; and 0 to 5 mole %, based on the equivalent total weight of the first and second diols, of a monofunctional compound; in which the equivalent ratio of the MPL diol to the unbranched diol is from 20:80 to 75:25.

It has been found that a cost-favourable (because of the saving of solvent) single-stage process in accordance with the invention can be carried out so that it is easily reproducible, if the characteristic number of the polyurethane-forming components is, as is preferred, at least 96 and not more than 100, and advantageously from 97 to 99. Within the 96 to 100 range, the product can have a low melting point and high strength. The characteristic number is defined as the number of NCO group equivalents per 100 OH group equivalents. It is therefore preferred that there should be an excess of OH groups or, at most, equivalent amounts of NCO and OH groups.It has been found that, when the OH groups are in excess, and even when temperatures up to 2500C occur during the strongly exothermic polyurethane formation, no side reactions, resulting in a product having a high melting point, taken place. It is therefore unnecessary to remove heat from the reaction.

The characteristic number should, advantageously, not be below 96 because, in such a case, the strength of the product is relatively low.

The reactants in the process of the invention include a diol having melting point-lowering characteristics and a molecular weight below 500. Suitable diols of this type have side chains and/or ether groups.

Examples of such compounds are neopentyl glycol, 2,2-diethylpropane-1, 3-diol diethylene glycol and 1,5-pentanediol. It is particularly preferred to use a mixture of two such diols, for example a combination of a branched glycol such as neopentyl glycol and an ether glycol such as diethylene glycol, thereby providing a third chain-lengthening diol.

The reactants used in this invention also include a second low molecular weight diol which assists the preparation of polyurethanes having good strength properties. Such diols should be unbranched and free of ether groups. Preferred examples of such compounds are ethylene glycol, 1,4-butylene glycol and hexamethylene glycol and they will generally have the formula HO-(CH2)2p -OH in which p is from 1 to 16. If desired, a mixture of two or more of these unbranched diols may be used. The low molecular weight diols assist in the preparation of polyurethanes having low melting points, e.g. below 200"C.

The equivalent ratio of the low molecular weight diol determines, to a large extend, the strength/hardness of the polyurethane. Polyurethanes produced using a high proportion of diisocyanate are relatively strong owing to the high urethane content, and the proportion of the MPL diol may be low. Conversely, soft polyurethanes with low melting point and good strength can be obtained when the MPL diol is the predominant component of the diol mixture.Although, in practice, it is difficult to determine the distinction between "soft" and "hard" polyurethanes, an arbitrary division between the two classes can be defined by a medium hardness range obtained when 50 to 70 parts by weight of diisocyanate are used per 100 parts of polyhydric alcohol, particularly when using an aliphatic diisocyante such as 1,6-hexamethylene diisocyanate with a polyester having a molecular weight of 1000 to 2000.

If necessary or desired, a small proportion of a monofunctional compound, containing one active hydrogen atom, may be included in the reactants used in this invention. The proportion must be below 5 mole %, based on the equivalent total weight of the first and second diols, and is preferably below 3 mole %.

If a monofunctional compound is present, it may not be especially desirable to use a mixture of two MPL diols. Suitable monofunctional compounds include amines, amides and alcohols. Examples of such compounds are dibutylamine, caprolactam and neopentyl alcohol.

The first diol which is used in the invention has a molecular weight of between 500 and 5000, preferably from 1000 to 2000. This compound is suitably a polyether, e.g. based on tetrahydrofuran, or a polyether, e.g.

based on adipic acid or capralactome. Diols or diol mixtures comprising ethylene glycol, prolylene glycol, butylene glycol neopentyl glycol or hexamethylene glycol are suitable diol constituents of the polyadipates.

Diisocyanates which can be used in the invention include aromatic diisocyanates such as 4,4'diphenylemethane diisocyanate and derivatives (such as carbodiimide-modified derivatives), ditolyl diisocyanate, cycloaliphatic diisocyanates such as isophorone diisocyanate,and preferably, aliphatic diisocyanates such as hexamethylene diisocyanate and 2,4,4-trimethylhexamethylene diisocyanate. A mixture of two or more diisocyanates may be used, as is also the case with the first diols.

The process of the invention can be carried out in one or a number of stages. The temperature at which the reaction is conducted is determined on the basis of whether a soft or a hard polyurethane is desired. For example, reactants containing at least 80 parts by weight of diisocyanate per 100 parts by weight of a polyester having a molecular weight of 2000 are heated to, for example, 50-60"C. With lower diisocyanate contents, the starting temperature may be up to 100"C (such a temperature is suitable for a mixture containing 30 parts by weight of diisocyanate per 100 parts by weight of a polyester having a molecular weight of 2000).

The reaction is exothermic but, as indicated above, the high temperatures which can be involved at the end of the reaction need not effect the quality of the products or the reproducibility of the process.

Accordingly, the reaction can be allowed to proceed to completion in a short time. Further, because the products have relatively low viscosity at the high temperatures which may be involved, e.g. 200 to 240 C, they may be subsequently processed by the simple expedient of pouring them out of the reaction vessel. It is found that the reheating which has frequently been necessary in the prior art to remove residual isocyanate groups can be avoided.

Typical reaction times for a batch containing an excess of diisocyanate over polyester, after sufficient heat has been supplied for the mixture to reach a temperature of 50"C, are 15 to 20 minutes.

While a single-stage process may be most convenient for carrying out the process of this invention, in which case the individual components, which may be liquid or solid, are introduced into the reaction vessel in any desired sequence, the reaction may also be carried out in a conventional 2-stage process. The polyurethanes may be prepared discontinuously or continuously, for example in a reaction extruder.

The polyurethanes produced in accordance with the invention, can have high tensile strength, for example more than 10, and often more than 15, N/mm2 in accordance with DIN 53371. Such values make the products of this invention particularly suitable, for example between non-woven and woven fabrics, i.e. as fusible adhesives in interlinings.

The following Examples 1 to 3 illustrate the process of this invention; Example 4 is comparative.

Example 1 A mixture of 34.6 kg of a polybutylene glycolethylene glycol adipic acid ester (OH number 56; molecular weight 2000), 27.7 kg of hexamethylene diisocyanate, 6.0 kg of neopentyl glycol 4.0 kg of diethylene glycol and 5.0 kg of 1,4-butylene glycol (this mixture has a characteristic number of 98) was reacted, in a single-stage process, in a reaction vessel and heated to 60"C while stirring. Owing to the exothermic reaction, the temperature rose to 200"C in about 20 minutes. At this temperature, the polyurethane was poured out into flat containers.

Examples 2 to 4 3 further polyurethanes were prepared in accordance with the general procedure of Example 1, varying the amounts of the ingredients as shown in Table I. Diol 3 is diethylene glycol in Example 1 and 1,6-hexanediol in Examples 2 and 3.

TABLE I Example 1 2 3 4 PG-EG 34.6 20.0 20.0 20.0 ester (kg) Neopentyl 6.0 2.08 1.66 4.37 Glycol (kg) 1,4-Butylene 5.0 1.80 2.16 4.5 Glycol (kg) Diol 3 (kg) 4.0 2.69 2.69 Hexamethylene 27.7 12.0 12.0 16.8 Diisocyanate (kg) Characteristic 98 98 98 98 Number Various tests were then conducted to determine the properties of the products of the above Examples.

These results are given in Table II. Melting ranges (MP) were determined in degrees C on a Kofler heating bench. Melting indexes (MI) were determined in a g/min. at 2.16 kp at 140"C. Tensile strength (in N/mm2 by DIN 53371), elongation at breach (in % by DIN 53371) and tearing resistance (in N/mm by DIN 53356) were determined on 0.3 mm thick films formed by extruding the polyurethane TABLE II Example 1 2 3 4 MP 115-125 120-130 120-130 120-130 Ml 40/10 44/10 53/10 28/10 Tensile 11.5 16.8 17.2 Strength Elongation 560 650 580 at Break Tearing 43.4 48.5 57.0 Resistance The polyurethanes of Examples 2 and 3 which contain a substantially smaller proportion of neopentyl glycol than that of Example 1, have higher strength values. Their melting points are only slightly higher, however.

The following Examples 5 and 6 illustrate uses of the products of this invention; Example 7 is comparative.

Example 5 The grannular polyurethane material of Example 1 was melted in conventional manner in an extruder and spun through suitable nozzles. The spun polyurethane non-woven which wasformed, having a weight of 15 glum2, was applied to a random-laid support non-woven with a weight of 30 g/m2.

The fixing interlining obtained in this way was ironed, using a plate press, onto a raincoat fabric whose inner side (against which fixing was effected) was finished over its entire area with a polyurethane coating.

The ironing was conducted at a pressure of 350 mbar for 10 seconds and at various plate temperatures, of 100 to 160 C. The following separation values (in N/5 cm strip width) were obtained when measured according to DIN E 54310, the non-woven breaking down in all cases: Plate Temperature (C) 100 120 140 150 160 Separation value (N/5 cm) 6.9 6.9 13.5 17.1 13.2 After dry cleaning in perchloroethylene according to DIN 54203, the bond was sufficiently strong for itto be the non-woven which broke down in the separation test. The same applied after washing at 600C according to DIN 53920.

After fixing on a Rolomatic continuous press (setting 120"C) the adhesion in the primary state and after dry cleaning in perchloroethylene or after washing at 600C was sufficiently good and the non-woven broke down.

The surface of the raincoat fabrics remained flawless in all cases, i.e. completely smooth and silkily lustrous.

Examples 6and 7 To a non-woven viscose fibre fabric (weight 30 g/m2) there were evenly applied 30 g/m2 of a powder having a particle size 200 to 4001lm which had been obtained by grinding granules of the polyurethane of Examples 1 and 4, respectively. The powders were sintered on for 3 minutes at 170"C. The samples were thereafter pressed together with (A) a woven polyester-cotton fabric and (B) a woven cotton poplin fabric, for 12 seconds at 1 So0C and a pressure of 350 g/cm2. The samples obtained were tested in the following 4 tests: (1) Primary adhesion to woven polyester-cotton fabric (2) Adhesion after cleaning with perchloroethylene (3) Primary adhesion to woven cotton poplin fabric (4) Adhesion afterwashing at600C.

The strength of adhesion values in Table Ill are in N/5 cm strip width.

TABLE Ill Test 1 2 3 4 Example 1 product 14.0 15.0 26.0 21.0 Example4product 5.8 4.1 11.7 8.2 As can be seen, the polyurethane prepared according to Example 1 which, in addition to neopentyl glycol, contains diethylene glycol as a starting component, gives higher adhesive strength values in all cases than in the case of the samples with a polyurethane of Example 4 as adhesive.

Claims (14)

1. A process for preparing a polyurethane, which comprises reacting together, in the absence of a solvent, a diisocyanate; a first diol, having an average molecular weight of between below 500, one of which has a melting point-lowering characteristics and another of which is unbranched and free of ether groups; and 0 to 5 mole %, based on the equivalent total weight of the low molecular weight diols, of a monofunctional compound; in which the equivalent ratio of the melting point-lowering diol to the unbranched diol is from 20:80 to 75:25.

2. A process according to claim 1 in which the melting point-lowering diol is neopentyl glycol, 2,2-diethylpropylene glycol, diethylene glycol or 1,5-pentanediol.

3. A process according to claim 1 or claim 2 in which the unbranched diol is ethylene glycol, butylene glycol or hexamethylene glycol.

4. A process according to any preceding claim in which the polyurethane-forming components have a characteristic number of from 97 to 99.

5. A process according to any preceding claim in which the diisocyanate is aliphatic.

6. A process according to claim 5 in which the diisocyanate is hexamethylene diisocyanate.

7. A process according to any preceding claim in which the first diol is a polyester having an average molecular weight of from 1000 to 2000.

8. A process according to any preceding claim in which the monofunctional compound is present.

9. A process according to claim 8 in which the monofunctional compound is an amine, or amide or an alcohol.

10. A process according to claim 9 in which the monofunctional compound is dibutylamine, caprolactam or neopentyl alcohol.

11. A process according to claim 1 substantially as described in any of Examples 1 to 3.

12. A fusible interlining comprising a fabric coated with a polyurethane obtained by a process according to any preceding claim.

13. An interlining according to claim 12 in which the polyurethane is in the form of a spun non-woven.

14. An interlining according to claim 13 in which the spun non-woven comprises from 33 to 67% by weight thereof of a thermofixible polyester having a melting range comparable to that of the polyurethane.